1. Technical Field
The present invention relates to an improvement in an electric power steering control system in which an electric motor generates torque for assisting steering torque generated by driver""s steering wheel manipulation.
2. Background Art
A power steering control system is used as a system in which driving force of another power source (such as a hydraulic pump or an electric motor) assists driver""s steering wheel manipulation to reduce driver""s force necessary for manipulation of the steering wheel and facilitate manipulation of the steering wheel. In the following description, a system in which an electric motor is used as the foregoing another power source is referred to as an electric power steering control system in order to distinguish the system from other systems.
An example of an electric power steering control system, FIG. 10 shows a construction of a system described in Japanese Patent Application No. 016026/2000 previously filed by the applicant. In the drawing, reference numeral 10 is an electric motor (hereinafter simply referred to as motor) for driving the steering system (not shown). Numeral 1 is a steering torque detector (which is referred to as steering torque detecting means) for detecting a steering torque generated by driver""s steering wheel manipulation (not shown) and outputs a steering torque signal. Numeral 2 is a steering torque controller (which is referred to as steering assist controlling means) for computing a steering assist torque signal on the basis of the steering torque signal. Numeral 17 is a return torque compensator which outputs a steering wheel return assist torque signal for generating a torque of the motor 10 in the direction of returning the steering wheel to a starting point on the basis of a road surface reaction torque signal which is an output of a road surface reaction torque detector 15. Numeral 5 is a motor speed detector, numeral 3 is a damping compensator which receives a motor speed signal and compensates its damping, numeral 4 is an inertia compensator, numeral 6 is a motor acceleration detector, numeral 7 is a motor current determiner, numeral 9 is a motor drive, numeral 11 is a motor current detector, numeral 12 is a first adder, numeral 13 is a second adder, and numeral 14 is a speed detector.
Numeral 15S is a road surface reaction torque detector provided with a low-pass filter. The road surface reaction torque detector 15S computes a road surface reaction torque signal on a S/W of a microcomputer on the basis of a steering torque signal which is an output of the steering torque detector 1, a motor acceleration signal which is an output of the motor acceleration detector 6, and a motor current value outputted by the motor current detector 11. Then, the road surface reaction torque detector 15S outputs the road surface reaction torque signal. FIG. 12 shows a diagram for explaining the processing operation of the road surface reaction torque detector 15S in the computation, and the computation is described later in detail.
Operation of the electric power steering control system of FIG. 12 is described below with reference to a flowchart of FIG. 11.
First, in Step S301, a steering torque signal detected by the steering torque detector 1 is read and stored in a memory. Next, in Step S302, a motor speed signal detected by the motor speed detector 5 is read and stored in the memory. In Step S303, the motor acceleration detector 6 differentiates the motor speed signal, and a motor acceleration signal is obtained and stored in the memory. In Step S304, a motor current signal is read and stored in the memory.
Then, in Steps S305 to S306, the following computation is conducted in the road surface reaction torque detector 15S, and a road surface reaction torque signal is obtained.
First, in Step S305, a stationary reaction force signal Txe2x80x2rea-est is obtained from the fogoing Equation (1) using a steering torque signal Tsens, a motor acceleration signal dxcfx89 equivalent to a rotational acceleration of the steering shaft, and a motor current signal Imtr.
Txe2x80x2rea-est=Tsens+Ktxc2x7Imtrxe2x88x92Jxc2x7dxcfx89xe2x80x83xe2x80x83(1)
where:
Kt: torque constant of the motor (computed in terms of steering shaft)
J: moment of inertia of the steering mechanism
Next, in Step 306, the low-pass filter arranged in the road surface reaction torque detector 15S conducts a primary filter computation as shown in the following Equation (2) to obtain a road surface reaction torque signal Trea-est, and this road surface reaction torque signal Trea-est is stored in the memory.
dTrea-est/dt=xe2x88x92Trea-est/T1+Txe2x80x2rea-est/T1 xe2x80x83xe2x80x83(2)
where: T1 is a time constant of a primary filter in Equation (2), and is established so that a cutoff frequency fc=1/(2xcfx80xc2x7T1) may be in the range of 0.05 Hz to 1.0 Hz.
Next, in Steps S307 to S308, in the steering torque controller 2, the steering torque signal is passed through a phase compensator and phase-compensated, mapping operation is conducted with respect to the phase-compensated steering torque signal, and a steering assist torque signal is obtained and stored in the memory.
In Step S309, in the return torque compensator 17, mapping operation is conducted for the foregoing road surface reaction torque signal Trea-est, and a steering wheel return assist torque signal is obtained and stored in the memory.
In Step S310, in the damping compensator 3, a damping compensation signal is obtained by multiplying the motor speed signal and the proportional gain and the product is stored in the memory.
In Step S311, in the inertia compensator 4, an inertia compensation signal is obtained by multiplying the motor acceleration signal and the proportional gain and is stored in the memory.
Next, advancing to Step S312, the first adder 12 adds the steering assist torque signal, steering wheel return assist torque signal, damping compensation signal, and inertia compensation signal obtained in the foregoing Steps S308 to S311, thus a target torque is obtained and stored in the memory.
In Step S313, in the motor current determiner 7, a target current is obtained by multiplying the target torque obtained in the foregoing step S312 by a gain, and the target current is stored in the memory. The gain obtained at this time is an inverse (reciprocal) of the torque constant of the motor 10 computed in terms of steering shaft.
The foregoing Steps S301 to S313 are repeated.
Described below is the reason why it is possible to detect the road surface reaction torque from the foregoing Equation (1) and Equation (2).
The equation of motion of the steering mechanism is expressed by the following Equation (3).
Jxc2x7dxcfx89s/dt=Thdl+Tmtrxe2x88x92Tfricxe2x88x92Treactxe2x80x83xe2x80x83(3)
where:
dxcfx89s/dt: rotational acceleration of the steering shaft
Thdl: steering torque
Tmtr: motor output torque (computed in terms of steering shaft)
Tfric: friction torque in the steering mechanism
Treact: road surface reaction torque (computed in terms of steering shaft)
When solving the foregoing Equation (3) for the road surface reaction torque Treact, the following Equation (4) is obtained.
Treact=Thdl+Tmtrxe2x88x92Jxc2x7dxcfx89s/dtxe2x88x92Tfricxe2x80x83xe2x80x83(4)
Accordingly, the road surface reaction torque Treact is obtained by using the respective values of the steering torque, motor output torque, rotational acceleration of the steering shaft, and friction torque in the steering mechanism. In this respect, it is possible to use the steering torque signal Tsens as the steering torque Thdl, and it is possible to use a value obtained by multiplying the motor current signal Imtr by the torque constant Kt as the motor output torque Tmtr. It is also possible to use the motor acceleration signal dxcfx89 as the rotational acceleration of the steering shaft (d xcfx89s/dt). After all, it becomes possible to detect the road surface reaction torque excluding influence of the friction torque Tfric in the steering mechanism from the foregoing Equation (1).
On the other hand, the friction torque Tfric acts as a relay on the speed of revolution of the steering mechanism. It is well known that the relay can be equivalently expressed in the form of gain and phase by equivalent linearization method in the field of control engineering. Accordingly, when the gain and phase of the stationary reaction force signal Txe2x80x2rea-est detected in the foregoing Equation (1) are regulated by the primary filter in the foregoing Equation (2), the road surface reaction torque signal Txe2x80x2rea-est is obtained.
That is to say, the primary filter (low-pass filter) is used as the most popular method for regulating the gain and phase as shown in FIG. 12. The range in which the gain and phase can be regulated by the primary filter is a frequency range not lower than the cutoff frequency. When establishing the cutoff frequency in the range of 0.5 to 1 times the frequency to be regulated, the gain can be regulated within the range of approximately 1 to 0.5 times and the phase can be regulated within the range of 0 to 20 deg. Thus, the influence of the friction torque can be cancelled in most cases. The steering frequency generally used in vehicles is in the range of approximately 0.1 to 1 Hz. That is, when establishing the cutoff frequency in the range of 0.5 to 1 times the foregoing steering frequency, i.e., approximately 0.05 Hz to 1 Hz, it is possible to cancel the influence of the friction torque. In addition, the specific cutoff frequency is established aiming toward a steering frequency, on which control based on the detected road surface reaction torque signal is desired, that works most effectively.
As described above, in the power steering system of FIG. 12, the influence of the term (Jxc2x7dxcfx89s/dt) which is equivalent to the inertia of the motor, increases in proportion to square of the frequency, while the primary filter is used as the low-pass filter of the road surface reaction torque detector. As a result, the influence of the inertia of the motor increases in proportion to the frequency components of the force of manipulating the steering wheel as shown in the following Equation (5):
Jxc2x7f2/(T1xc2x7f+1)≈Jxc2x7f/T1 xe2x80x83xe2x80x83(5)
Therefore, an error in the term which is equivalent to the inertia of the motor due to detection error of the rotational acceleration of the steering shaft (dxcfx89s/dt) or estimation error of the moment of inertia (J) of the steering mechanism increases in proportion to the steering wheel manipulation. As a result, a problem exists in that when manipulating the steering wheel in a quick cycle which includes a lot of high frequency components (hereinafter referred to as high frequency steering), the motor generates unnatural steering wheel return torque and the steering wheel becomes unusually heavy.
The present invention was made to resolve the above-discussed problems and has an object of providing a power steering system in which any unnatural steering wheel return torque is not generated and the steering wheel does not become unusually heavy even when conducting a steering wheel manipulation under high-frequency.
An electric power steering control system according to the invention comprises:
an electric motor which generates a torque for assisting a steering torque generated by driver""s steering wheel manipulation;
steering torque detecting means for detecting the steering torque;
motor current detecting means for detecting a current flowing in the motor; and
first road surface reaction torque means for obtaining a road surface reaction torque detection value by passing a value obtained by adding the steering torque and a motor torque computed in terms of steering shaft from the motor current through filters formed by plural stages of primary low-pass filters connected in series.
As a result of such construction, even when conducting a steering wheel manipulation containing high frequency components, any unusually large steering wheel return torque is not generated. Thus, it is possible to achieve a power steering control system by which a driver can drive his vehicle without feeling something like difficulty in adapting himself to the power steering.
Another electric power steering control system according to the invention comprises:
an electric motor which generates a torque for assisting a steering torque;
steering torque detecting means for detecting the steering torque;
motor current detecting means for detecting a current flowing in the motor;
rotational acceleration detecting means for detecting a rotational acceleration of the electric motor; and
second road surface reaction torque detecting means for obtaining a road surface reaction torque detection value by passing a value obtained by subtracting a motor inertia torque computed in terms of steering shaft from an output of the rotational acceleration detecting means from a value obtained by adding the steering torque and a motor torque computed in terms of steering shaft from the motor current through filters formed by plural stages of primary low-pass filters connected in series.
As a result of such construction, even when conducting a steering wheel manipulation containing high frequency components, any unusually large steering wheel return torque is not generated. Thus, it is possible to achieve a power steering control system by which a driver can drive his vehicle without feeling something like difficulty in adapting himself to the power steering.
It is preferable that the electric power steering control system is provided with a limiter for limiting the value obtained by subtracting the motor inertia torque computed in terms of steering shaft from an output of the rotational acceleration detecting means from the value obtained by adding the steering torque and a motor torque computed in terms of steering shaft from the motor current not to exceed a predetermined value.
As a result of such construction, even when conducting a steering wheel manipulation containing high frequency components, any unusually large steering wheel return torque is not generated. Thus, it is possible to achieve a power steering control system by which a driver can drive his vehicle without feeling something like difficulty in adapting himself to the power steering.
It is preferable that the plural stages of primary low-pass filters include at least one filter whose time constant is not less than 0.05 Hz and not more than 1 Hz and at least one filter whose time constant is not less than 1 Hz and not more than 3 Hz.
As a result, the driver can drive without feeling something like difficulty in adapting himself to the power steering.
A further electric power steering control system according to the invention comprises:
an electric motor which generates a torque for assisting a steering torque;
steering torque detecting means for detecting the steering torque;
motor current detecting means for detecting a current flowing in the motor;
a limiter for limiting the value obtained by adding the steering torque and a motor torque computed in terms of steering shaft from the motor current not to exceed a predetermined value; and
third road surface torque detecting means for obtaining a road surface reaction torque value by passing the mentioned value through a low-pass filter.
As a result of such construction, even when conducting a steering wheel manipulation containing high frequency components, any unusually large steering wheel return torque is not generated. Thus, it is possible to achieve a power steering control system by which a driver can drive his vehicle without feeling something like difficulty in adapting himself to the power steering.
A still further electric power steering control system according to the invention comprises:
an electric motor which generates a torque for assisting a steering torque;
steering torque detecting means for detecting the steering torque;
motor current detecting means for detecting a current flowing in the motor;
rotational acceleration detecting means for detecting a rotational acceleration of the electric motor;
a limiter for limiting a value obtained by subtracting a motor inertia torque computed in terms of steering shaft from the rotational acceleration from a value obtained by adding the steering torque and a motor torque of the electric motor computed in terms of steering shaft from the motor current not to exceed a predetermined value; and
fourth road surface reaction torque detecting means for obtaining a road surface reaction torque detection value by passing the mentioned value through a low-pass filter.
As a result of such construction, even when conducting a steering wheel manipulation containing high frequency components, any unusually large steering wheel return torque is not generated. Thus, it is possible to achieve a power steering control system by which a driver can drive his vehicle without feeling something like difficulty in adapting himself to the power steering.
A method for controlling an electric power steering control system according to the invention comprises the steps of:
detecting a steering torque generated by steering wheel manipulation;
detecting a current of an electric motor which generates a torque for assisting the steering torque;
detecting a rotational acceleration of the electric motor; and
detecting a road surface reaction torque for obtaining a road surface reaction torque detection value by passing a value obtained by subtracting a motor inertia torque computed in terms of steering shaft from the rotational acceleration from a value obtained by adding the steering torque and a motor torque computed in terms of steering shaft from the motor current through primary low-pass filters formed by plural stages of primary low-pass filters connected in series.
As a result of such control method, even when conducting a steering wheel manipulation containing high frequency components, any unusually large steering wheel return torque is not generated. Thus, it is possible to achieve a power steering control system by which a driver can drive his vehicle without feeling something like difficulty in adapting himself to the power steering.
Another method for controlling an electric power steering control system according to the invention comprises the steps of:
detecting a steering torque generated by steering wheel manipulation;
detecting a current of an electric motor which generates a torque for assisting the steering torque;
detecting a rotational acceleration of the electric motor;
limiting a value obtained by subtracting a motor inertia torque computed in terms of steering shaft from the rotational acceleration from a value obtained by adding the steering torque and a motor torque computed in terms of steering shaft from the motor current not to exceed a predetermined value; and
detecting a road surface reaction torque for obtaining a road surface reaction torque detection value by passing the mentioned value through a low-pass filter.
As a result of such control method, even when conducting a steering wheel manipulation containing high frequency components, any unusually large steering wheel return torque is not generated. Thus, it is possible to achieve a power steering control system by which a driver can drive his vehicle without feeling something like difficulty in adapting himself to the power steering.
It is also preferable that the electric power steering control system is provided with a limiter for limiting the value obtained by adding the steering torque and the motor torque computed in terms of steering shaft from the motor current not to exceed a predetermined value.
As a result of such construction, even when conducting a steering wheel manipulation containing high frequency components, any unusually large steering wheel return torque is not generated. Thus, it is possible to achieve a power steering control system by which a driver can drive his vehicle without feeling something like difficulty in adapting himself to the power steering.